1. Field of the Invention
The present invention relates to the design of semiconductor light-emitting devices. More specifically, the present invention relates to novel semiconductor light-emitting devices with highly reflective ohmic contact.
2. Related Art
Solid-state lighting is expected to bring the next wave of illumination technologies. High-brightness light-emitting diodes (HB-LEDs) are emerging in an increasing number of applications, from serving as the light source for display devices to replacing light bulbs for conventional lighting. Typically, cost, efficiency, and brightness are the three foremost metrics for determining the commercial viability of LEDs.
An LED produces light from an active region, which is “sandwiched” between a positively doped layer (p-type doped layer) and a negatively doped layer (n-type doped layer). When the LED is forward-biased, the carriers, which include holes from the p-type doped layer and electrons from the n-type doped layer, recombine in the active region. In direct band-gap materials, this recombination process releases energy in the form of photons, or light, whose wavelength corresponds to the energy band-gap of the material in the active region.
Depending on the selection of the substrate and the design of the semiconductor layer stack, an LED can be formed using two configurations, namely the lateral-electrode (electrodes are positioned on the same side of the substrate) configuration and the vertical-electrode (electrodes are positioned on opposite sides of the substrate) configuration. FIGS. 1A and 1B illustrate both configurations, where FIG. 1A shows the cross-section of a typical lateral-electrode LED and FIG. 1B shows the cross-section of a typical vertical-electrode LED. Both of the LEDs shown in FIGS. 1A and 1B include a substrate layer 102, an n-type doped layer 104, a multi-quantum-well (MQW) active layer 106, a p-type doped layer 108, a p-side electrode 110 coupled to the p-type doped layer, and an n-side electrode 112 coupled to the n-type doped layer.
The vertical-electrode configuration makes the packaging of the device easier. In addition, because the electrodes are located on opposite sides of the device, the device is more resistant to electrostatic discharge. Therefore, a vertical-electrode LED has a higher stability compared with a lateral-electrode LED. This is especially true for high-power, short-wavelength LEDs.
In order to extract light effectively from a high-power high-brightness LED, a flip-chip packaging technique is often adopted, in which the p-side electrode is used as a highly reflective surface to reflect light to the opposite side of the device. The presence of a light reflector increases the light extraction efficiency of the LED. FIG. 2 illustrates an exemplary structure of a flip-chip packaged vertical LED with the p-electrode as a reflector. From top down, FIG. 2 shows an n-side electrode 202, an n-type doped layer 204, an active layer 206, a p-type doped layer 208, a p-side electrode 210 which also acts as a reflector, and a supporting substrate 212. The arrows in dashed lines show the direction of the current flow, and the short arrows pointing upward show the direction of the extracted light. Note that unlike laser devices, in which emitted light is guided and propagates in a well-defined direction, the light emitted in an LED propagates omni-directionally. Hence, the reflector at the bottom of the device is essential in increasing the light extraction efficiency.